1. Field of the Invention
This invention relates generally to semiconductor processing, and more particularly to a chemical mechanical polishing system utilizing electrochemistry and to methods of using the same.
2. Description of the Related Art
Conventional chemical mechanical polishing (xe2x80x9cCMPxe2x80x9d) is, as the name implies, an amalgam of chemical and mechanical processes used to planarize wafer surfaces in semiconductor fabrication. In most conventional CMP processes, a quantity of slurry is deposited on a table or platen of a CMP machine or xe2x80x9cpolisher.xe2x80x9d The table has a polish pad that is composed of a compliant or semi-compliant material that partially conforms to the varied topography of a wafer film. Thereafter, the semiconductor wafer is brought into contact with the slurry and the polish pad and relative motion is provided between the wafer and the polish pad.
Conventional slurries normally contain several components, such as one or more types of abrasive particles, a stabilizer that is designed to keep the abrasive particles from going into solution, and one or more oxidizing agents. In the CMP of various metals used in semiconductor processing, such as tungsten, the oxidizing agent of the slurry reacts with the tungsten to form a passivation layer of metal oxide. In the case of tungsten, the passivation layer consists of soft WOx. The WOx prevents the transport of additional oxidizing chemicals deeper into the tungsten underlying the WOx. Portions of the WOx film forming on the higher elevation areas of the tungsten film will be easily removed by the mechanical abrasive action of the abrasive particles in the slurry. As the WOx is removed, additional fresh metal is exposed which simultaneously begins to react chemically with the oxidizing agent to reform additional WOx. In the lower elevation regions of the tungsten film, the WOx film forms a soft passivation layer that resists chemical etching as noted above, and because of its lower elevation, undergoes very minimal mechanical abrasion from the slurry. Eventually, the higher elevation surfaces are abraded and chemically etched down to the same level as the lower regions, and thereafter the combined passivation and abrasion processes continue to cycle until a desired polish end point is reached. The same types of chemical and mechanical mechanisms occur during CMP of the other metals, such as titanium, titanium nitride, copper, etc.
Successful CMP processing is dependent upon a number of factors, such as predictable slurry composition and predictable polish time periods. Uniform slurry composition over time ensures that wafers in a given lot and lot-to-lot are polished at the same rate. As the CMP process is consumptive of various slurry constituents, particularly the oxidizer components, it is necessary to replenish the slurry mixture frequently. Sometimes, the need for replenishment stems from the propensity for one or more of the slurry constituents to decompose over time. Hydrogen peroxide as an oxidizer is an example of such a rapidly decomposing constituent. The need for frequent replenishment, of course, increases the overall cost of the CMP process and can lead to variations in the slurry composition used for, and therefore the polish rates of, successive wafers in a given lot or for successive lots.
Predictable polish time periods again ensure that the polished film is planarized according to recipe and that process variations between successive wafers are minimal. However, conventional CMP processes sometimes do not proceed according to specified time periods. Process aborts are a leading cause of such variations. The causes for aborts are legion and include machine malfunction and slurry composition deviations to name just a few. Regardless of the exact cause, an abort usually results in the termination of the mechanical rotation of the wafer, or polish pad, depending on the tool type. During a normal CMP run, a wafer is exposed to a slurry and polished on a CMP machine for a preselected time period or until some other preselected endpoint is reached. The slurry is allowed to dwell, if at all, on the wafer surfaces for only preselected and relatively short time periods. However, during an abort, the chemical activity associated with the CMP slurry may continue, resulting in a static etch of the surfaces of the wafer exposed to the slurry. This can result in unwanted and substantial etch attack of various structures on the wafer.
Another drawback associated with the conventional slurries is the need for the inclusion of a corrosion inhibitor in order to prevent a so-called xe2x80x9cseamingxe2x80x9d or xe2x80x9ccoringxe2x80x9d of tungsten lines and vias. Corrosion inhibitors add to the cost of the slurry composition.
In many cases, the chemical reactivity associated with certain slurry constituents, while desirable from a chemical mechanical polishing standpoint are nevertheless undesirable from the standpoint of the corrosion incurred by various, particularly steel, components of the CMP machine. Ammonium chloride, which is commonly used as a stabilizer, is highly corrosive to steel and thus limits the life span of exposed CMP machine steel parts. Steel corrosion can also contribute to corrosion-related particulate contamination, which has the potential to degrade product yields.
The present invention is directed to overcoming or reducing the effects of one or more of the foregoing disadvantages.
In accordance with one aspect of the present invention, a device for planarizing a surface of a semiconductor workpiece is provided that includes a table for holding a quantity of an electrically conducting solution thereon. A member is included for holding the semiconductor workpiece such that the surface is in contact with the solution and operates as a working electrode. The member has a first conductor for establishing electrical connection with the semiconductor workpiece. A counter electrode is provided for making electrical connection with the solution and a reference electrode is provided for making electrical connection with the solution with a known electrode potential. A power source is operable to control the electric potential between the working electrode and the counter electrode.
In accordance with another aspect of the present invention, a method of planarizing a surface of a semiconductor workpiece is provided that includes contacting the surface with a quantity of an abrasive slurry and applying a potential between the surface and the slurry that is at least as high as the redox potential of the surface in order to oxidize the surface. The oxidized surface is abraded by providing relative movement between the slurry and the oxidized surface.
In accordance with another aspect of the present invention, a method of planarizing a surface of a semiconductor workpiece is provided that includes applying an electrically conducting solution to a table having an abrasive surface and contacting the workpiece surface with the electrically conducting solution. A potential is applied between the workpiece surface and the electrically conducting solution that is at least as high as the redox potential of the surface in order to oxidize the surface. The oxidized surface is abraded by providing relative rubbing movement between the abrasive surface of the table and the oxidized surface.